1. Field of the Invention
Technology of electric superconductors. The significance of materials having superconducting characteristics has been increasing more and more in recent times. The discovery of new superconducting materials, particularly of the rare earth/Ba/Cu/O type led to a considerable expansion in possible applications for superconductors since these materials already become superconducting at temperatures above 50K.
The invention relates to the further development and improvement of products from a ceramic high-temperature superconductors, taking into account the requirements of large-scale industrial production.
In particular, it relates to a method for producing a wire- or band-shaped ceramic high-temperature superconductor based on superconducting ceramics of the (Y,SE)Ba.sub.2 Cu.sub.3 O.sub.6.5+y class with SE=rare earth metal and 0&lt;y&lt;1.
2. Discussion of Background
It has been found that ceramic high-temperature superconductors of the SEBa.sub.2 Cu.sub.3 O.sub.6.5+y class (SE=rare earth metal, 0&lt;y&lt;1) have highly anisotropic characteristics in every regard. This is connected with the crystal structure (perovskite lattice) and particularly applies to i the critical current density, j.sub.c (current carrying capability). The grain boundaries in polycrystallized ceramics limit the critical current densities to small values so that these materials are not suitable for most technical applications at the boiling temperature of liquid nitrogen (T=77K). Hitherto, critical current densities, j.sub.c, up to a maximum of 1000 A/cm.sup.2 have been measured for sintered samples of polycrystalline YBa.sub.2 Cu.sub.3 O.about..sub.7. Applications in, for example, magnet coils, however, require current densities which are higher by a factor of about 100.
The above shows that, compared with erratically and statistically arbitrarily arranged crystallites, much higher critical current densities can be expected from an oriented crystallite structure. The highest current densities, j.sub.c, of more than 10.sup.5 A/cm.sup.2 were observed in thin superconductor layers epitaxially grown on SrTiO.sub.3 monocrystals. However, this method requires extensive monocrystals as substrates and cannot be used for producing wires (multiple filaments/and bands of great length).
Textures with preferred orientation of the crystallite axes can also be achieved by hot pressing and hot extrusion of ceramic powders. However, nothing is known about the current carrying capability of such products. In addition, these methods cannot really be used for producing wire- or band-shaped superconductors.
It is known that calcined powders, for example of the YBa.sub.2 Cu.sub.3 O.about..sub.7 compound, can have a needle-shaped or plate-shaped particle morphology which originates from growth anisotropies. The short axis of these particles crystallographically corresponds to the c axis of the lattice. The long axes of the particles reproduce a- or b-axis of the lattice and these directions can accommodate distinctly higher critical currents than the c axis. Plate- or needle-shaped particles each consisting of a monocrystal can be obtained by grinding porous agglomerates of oriented grains or by grinding and screening or by separation from the other phases in a magnetic field at low temperatures (Meissner effect).
An effect which is called secondary recrystallization (also "giant grain growth") is known from the literature. In secondary recrystallization, grain growth occurs in a structure only in the case of a very small number of grains acting as nuclei. The remaining grains in the structure do not change very much until they are completely consumed by the growth of the nuclei. The nuclei can grow into grains of up to 1 mm. If the nuclei have a preferred orientation before the grain growth begins, secondary recrystallization will produce a structure having a corresponding texture. This phenomenon has been observed in, among other things, ceramic magnetic materials such as ferrites. In this connection, a ferrite powder was pressed in a magnetic field to form green compacts. After the sintering, secondary recrystallization produced a preferred direction of the grains which was apparently predetermined by the orientation of a few nuclei.
The following references relating to the background are quoted:
I-Wei Chen, et al., "Texture Development in YBa.sub.2 Cu.sub.3 O.sub.x by Hot Extrusion and Hot-Pressing", J.Am.Ceram. Soc. 70, December 1987, C-388 - C-390); PA0 S. Hayashi et al., "Growth of YBa.sub.2 Cu.sub.3 O.sub.9- Single Crystals from the High Temperature Solution", Japanese Journal of Applied Physics, Vol.26, No. 7, July 1987, pp. L1197-L1198; PA0 D. L Kaiser et al., "Growth of YBa.sub.2 Cu.sub.3 O.sub.x single crystals", Appl. Phys. Letters 51, 1987, 1040-1042; PA0 P. Murugaraj et al., "Preparation of highly oriented Polycristalline YBa.sub.2-y Cu.sub.3 O.sub.x Superconductors", Solid State Communications, Vol. 66, No. 7, 1988, pp. 735-738; PA0 S. Vieira et al., "A simple device for quick separation of high-Tc superconducting materials", J. Phys.E:Sci. Instrum. 20, 1978, 1292-1293; PA0 A. L. Stuijts, "Sintering of Ceramic Permanent Magnetic Material", Trans. Brit. Ceram. Soc. 55, 1956, 57-74.